The invention relates to a method of manufacturing a semiconductor device whereby a doping is provided in a semiconductor body through openings in a mask provided above a surface of the semiconductor body, whereupon a diffusion step is carried out such that the doping concentration in the semiconductor body rises in accordance with a desired profile in a lateral direction parallel to the surface.
Such a method is known inter alia from the patent U.S. Pat. No. 5,300,448. This describes a high-voltage transistor of the lateral DMOS type (LDMOS) in a comparatively thin silicon layer, provided on a buried oxide layer which covers the surface of a silicon crystal. To obtain an optimum resurf condition with a uniform distribution of the electric field strength, the drift region of the transistor is given a doping profile in which the doping concentration increases linearly from the source to the drain. The manufacture of this doping profile is achieved by means of a doping mask provided with openings whose diameters increase in a direction from the source to the drain and which are situated at a substantially constant pitch. The desired linear doping profile is formed by means of a diffusion step at high temperature and of long duration. It was found that the electric field exhibits major fluctuations already in the case of comparatively small deviations in the envisaged doping profile. These fluctuations in the electric field may give rise to impact ionization and hot carrier degradation, so that the quality of the transistor is impaired. The invention has for its object in general to provide a doping method whereby a smoother doping profile can be obtained than by the known method. The invention further has for its object to provide a semiconductor device for use at high voltages wherein degradation owing to local peaks in the electric field strength is avoided as much as possible.
According to the invention, a method of the kind described in the opening paragraph is for this purpose characterized in that a mask is used for obtaining said profile whose pitch between openings becomes smaller in said lateral direction at least within part of the doping profile. It was found from experiments that peaks in the concentration in the eventual doping profile are at least substantially avoided, and thus also peaks in the electric field distribution.
A major embodiment of a method according to the invention is characterized in that a doping profile is formed which changes gradually between a minimum doping concentration at one side and a maximum doping concentration at an opposed, second side, for which purpose a doping mask is used with a first partial region corresponding to a portion of the doping profile in which the doping concentration is lower than half the difference between the maximum value and the minimum value, while at least the openings in the first partial region of the doping mask have at least substantially equal widths and are situated at variable distances from one another, seen in the direction of the doping profile. Advantageously, the openings in the first partial region may be manufactured so as to have the smallest dimensions which are possible in the process used, so that it is possible to adjust the local concentration over a wide range of doping concentrations by means of the concentration of mask openings. A further embodiment of a method according to the invention, whereby the concentration is adjusted by means of the density of mask openings also for higher doping concentrations, is characterized in that a second partial region has openings of varying widths, openings situated next to one another being separated from one another by portions of the mask material of at least substantially equal widths. Preferably, the masked portions between the mask openings in the second partial region are provided so as to have the smallest dimensions which are possible in the process.
The invention may be used to advantage in various types of semiconductor devices for obtaining a doping profile adapted to the specific device. Special advantages are obtained in the formation of the drift regions in high-voltage transistors. A preferred embodiment of a method according to the invention is characterized in that the transistor is of the lateral DMOS type, with a source zone, a drain zone, and a drift region of a first conductivity type and a back-gate region of the opposed, second conductivity type situated between the source zone and the drift region, the doping concentration in the drift region increasing in the direction from the source zone to the drain zone. The transistor may be formed in a surface layer of a first conductivity type which merges via a pn junction into a subjacent region or substrate of the second conductivity type. In this case the drift region is preferably given a doping profile in which the concentration rises in the direction from the source to the drain proportionally to the square root of the distance. An embodiment in which the doping profile rises linearly with this distance is characterized in that the semiconductor body is formed by a silicon layer which is provided on a substrate of electrically insulating material.